High intensity focused ultrasound (“HIFU”) is a rapidly developing medical technology that relies on focusing acoustic waves to treat remote tissue sites inside the body without damaging intervening tissues. HIFU can be used to treat benign and malignant tumors, dissolve blood clots, enhance drug delivery to specific sites, and ablate brain tissue causing essential tremors. A key feature of HIFU is the ability to maintain a very thin margin between treated and untreated tissue. However, the position and extent of treatment can be sensitive to many factors, including blood perfusion, tissue properties, and nonlinear acoustic propagation. In order to ensure effective treatments and to avoid adverse effects from unintended tissue injury, it is necessary to accurately determine the three-dimensional acoustic field that will be delivered to the patient. While standard practices for characterizing diagnostic ultrasound are well established, the lack of analogous metrology techniques for therapeutic ultrasound remains an impediment to broader clinical acceptance of HIFU.
Because ultrasound consists of waves, it possesses several basic features of wave physics that are of practical utility. In particular, it is possible to reproduce a three-dimensional field from a two-dimensional distribution of the wave amplitude and phase along some surface transverse to the wave propagation. This principle is widely used in optics, and the corresponding process is termed “holography.” A similar approach is possible in acoustics. For acoustic pressure waves, amplitude and phase can often be measured directly with a pressure sensor, and a two-dimensional distribution of such measurements represents a hologram.
Mathematically, the hologram provides a boundary condition for the wave equation, thereby permitting the calculation of acoustic variables anywhere in three-dimensional space, including the surface of the ultrasound transducer itself. However, it can be difficult to characterize an acoustic field created by a given ultrasound transducer with a high degree of accuracy. This is because transducers can be characterized by various shapes, sizes, frequencies, operation modes, and output intensities. Many utilize an array of independent elements that can operate in both continuous-wave and pulsed modes. Corresponding acoustic fields can possess complex three-dimensional structures: aside from targeted focal regions, transducers frequently create parasitic foci and grating lobes, either due to details of the source or inhomogeneities in tissue. Standard approaches for characterizing the field structure of ultrasound sources are based on point-by-point hydrophone measurements in water. However, direct hydrophone measurement of HIFU pressures is challenging for two reasons: (1) high pressure amplitudes require large measurement bandwidths and can damage hydrophones; and (2) large treatment volumes in conjunction with multiple operation modes (such as phased-array steering of the acoustic beam) require a prohibitive number of discrete measurements. Because of these challenges, as well as the complexity of holography and the difficulty in getting reliable results, acoustic holography has not been widely adopted in therapeutic ultrasound systems.